In recent years, a power supply system composed by combining a solar battery and a storage battery has been used as a self-contained power supply or a power leveling mechanism thereof. For example, this power supply system supplies power generated by the solar battery to a load while charging the storage battery with surplus power in the daytime during which sunshine is abundant, and supplies necessary power to the load by the discharging from the storage battery in the daytime during which sunshine is scarce or the nighttime.
However, for example, when the sunshine in the daytime is abundant and the storage battery is kept supplied with surplus power, the storage battery may be overcharged, and it accelerates the degradation of the storage battery. Meanwhile, when sunshine is scarce, since the storage battery has to keep supplying power to the load, the storage battery may be overdischarged, and it also accelerates the degradation of the storage battery.
For the prevention of the degradation of the storage battery mentioned above, it is preferable to use the storage battery while constantly keeping the battery capacity within the range in which normal charging/discharging is possible. For example, Japanese Patent Application Laid-Open Publication No. 2010-063359 (Patent Document 1) describes a power supply system including a solar battery, a lead storage battery, a charge control device which charges the lead storage battery with power output from the solar battery, and a photosensor. The charge control device has a function of opening a charge circuit when the voltage of the storage battery rises to a first set voltage during the charging and closing the charge circuit to recharge the storage battery when the voltage of the storage battery drops to a second set voltage lower than the first set voltage. Also, when a charge current to the lead storage battery is equal to or smaller than a set current, the charge control device does not open the charge circuit and continues the charging even if the voltage of the storage battery rises to a voltage equal to or higher than the first set voltage, and when an illuminance value measured by the photosensor drops to a value equal to or smaller than a set value, the charge control device controls the lead storage battery to supply power to a load. In this manner, decrease in the storage battery capacity caused by insufficient charging or overdischarging of the storage battery can be suppressed.
Also, for example, Japanese Patent Application Laid-Open Publication No. 2008-097941 (Patent Document 2) describes a technique for a charge management system which manages the charging of a storage battery. The charge management system includes a discharge capacity acquiring unit which acquires a discharge capacity of the storage battery and a charging unit which charges the storage battery at a charge capacity in accordance with the discharge capacity acquired by the discharge capacity acquiring unit, and by summing up the charge and discharge currents, the storage battery can be charged at a charge capacity in accordance with the discharge capacity up to the point of time of charging.